The present invention relates to a printer and, more particularly, to a serial printer which is actuated by a trigger mechanism to print alphanumeric characters from type in a type wheel stack.
Known serial printers usually select characters for printing by shifting and turning a wheel or a type cylinder, on which the type is carried, about a vertical axis on a carriage. The carriage moves the width of a paper sheet which is to be typed upon. Since such a type cylinder has a substantial height and must be moved a substantial distance, the carriage on which it moves must be large in size. Contributing to the size of the mechanism are shift selection and type selection mechanisms of a complex nature, including electromagnets, clutches, cams, and levers, all of which are controlled by a complicated process. For these reasons, it has heretofore been difficult to produce a small-size, inexpensive serial printer.
The serial printer described in Japanese Patent Laid-Open Publication No. 57-109677 has gone part of the way towards solving the above problem. The mechanism for shifting a stack of type wheels which is shown there, however, is not simple, since it requires a cam shaft passing through the carriage for transmitting drive power to a shift cam on the carriage to shift the type cylinder, a second shaft for turning the type cylinder and a pair of detectors for detecting the shifted and the angular positions of the type wheel stack.
In the serial printer shown in Japanese Patent Laid-Open Publication No. 57-173188, a hammer and a stack of type wheels which face each other on either side of an interposed sheet of print paper are driven by independent step motors and effect shift selection in response to differences in intervals of feed of the hammer and of the type wheel stack as it is moved across printing positions. This serial printer is expensive, however, as it requires costly machine components, such as step motors, and a transmission mechanism which uses wires for shift selection. Furthermore, the control process for driving the two independent step motors is complex. This serial printer is not suitable for low current designs and, most particularly, cannot be battery-driven. Also, the step motors themselves are, as a general matter, not efficient sources of motive power.